1. Field of the Invention
The present invention relates to detectors of the passing of vehicles"" axles a sensor of which is positioned in a roadway. Such detectors are used, for example, to count the vehicles driving on a road or in association with a tollgate to automatically count the number of axles of a vehicle arriving at the barrier.
2. Discussion of the Related Art
A first family of detectors uses the mechanical crushing under the effect of the vehicle weight to sense the passing of an axle. Such detectors are, for example, mechanical, piezo-electric or optical detectors.
A second detector family avoids wearing problems associated with mechanical stress and uses the magnetic signature of a vehicle. More specifically, a winding is buried in the roadway and is excited by an A.C. reference frequency signal. The passing of a vehicle above the winding causes a variation in the magnetic field which translates as a variation of the signal frequency in the winding. The frequency variations can then be monitored to detect the passing of vehicles.
FIG. 1 shows in a very simplified top view a roadway 1 equipped with a magnetic detector of the passing of vehicles. This detector is essentially formed of a winding 3 buried in roadway 1, approximately perpendicular to the traffic direction. Both ends of winding 3 are connected to an excitation and measurement device 4 including, among others, an oscillator for exciting winding 3 and means for measuring a variation of the signal frequency in the winding.
A problem which arises with conventional magnetic detectors is due to the metallic mass of the vehicle. Indeed, this metallic mass tends to disturb measurements or, at least, to make the detection of the vehicle axles particularly difficult.
To ease the counting or detection of axles, it has already been provided to take advantage of the existence of wire meshes in the tires of a vehicle""s wheels. Such wire meshes are elements disturbing the magnetic field, which are very close to the roadway. However, the magnetic variations due to tires are disturbed by the rest of the metallic mass of the vehicle and by possible metallic structures in the roadway, which strongly alter the wanted signal-to-general signal ratio.
U.S. Pat. No. 5,614,894 provides for distinguishing the variations due to metallic masses from those due to the vehicle""s tires based on the direction of magnetic field variation. The solution described in this document is, however, particularly complex to implement since the wanted signal-to-general signal ratio is extremely low due to the vehicle""s metallic mass. In practice, the frequency variations to be measured are on the order of 10xe2x88x926. Making such measurements reliable is particularly difficult.
Other magnetic detectors are described, for example, in documents WO-A-0,058,926 and WO-A-0,058,927.
The present invention aims at overcoming the disadvantages of known vehicle axle magnetic detectors.
The present invention more specifically aims at providing a novel magnetic detector of the passing of vehicle wheels which is no longer disturbed by the magnetic mass represented by the very vehicle.
The present invention also aims at providing a detector which is particularly simple to form and which is compatible with products currently available for sale.
The present invention also aims at providing a solution compatible with conventional exploitation circuits. In particular, the present invention aims at providing a detector which enables providing the same type of information as conventional detectors.
A feature of the present invention is to provide a concentration of the magnetic field of the coil buried in the roadway to prevent dispersal of the field lines towards the entire vehicle. More specifically, the present invention brings the size (width) of the detector winding down to the scale of the track of a vehicle wheel. For this purpose, the present invention provides associating with the sensor coil a ferrite structure. To fulfill the objective of bringing the size of the structure down to the scale of a wheel track, the winding and the ferrite structure are inscribed within an elongated surface which, moreover, is of large dimension (generally one or several meters long). A difficulty then is that the forming of an elongated ferrite structure of large dimension currently is difficult, or at least, relatively expensive. According to the present invention, small ferrite elements that are placed end to end in the large dimension of the structure to be obtained are preferably used. Standard ferrite elements, easily available for sale, can thus be used to form a structure of great length. To apply the present invention to magnetic sensors in roadways, the fact of providing a discontinuous ferrite structure (formed of elements of small size placed side by side) does not adversely affect the result.
More specifically, the present invention provides a magnetic sensor of the passing of at least one vehicle wheel adapted to being positioned on or in a roadway, including a conductive winding of at least one spiral and a ferrite structure inscribed in an elongated surface, the conductive element being wound around a portion of the ferrite structure, parallel to its greater dimension.
According to an embodiment of the present invention, the ferrite structure is positioned so that at least one air-gap is directed towards the roadway surface.
According to an embodiment of the present invention, the ferrite structure has an E-shaped cross-section, the conductive winding being wound around the central branch of the E.
According to an embodiment of the present invention, the ferrite structure has a U-shaped cross-section, the conductive winding being wound around the bottom of the U or on one of the sides.
According to an embodiment of the present invention, the ferrite structure has a length ranging between 2 and 10 cm and a length greater than 10 cm.
According to an embodiment of the present invention, the ferrite structure is formed of a succession of elements of small dimensions put end to end in the large dimension of the structure.
According to an embodiment of the present invention, the vehicle wheel magnetic detector includes a sensor of the above type and an excitation and measurement exploitation circuit.
According to an embodiment of the present invention, the circuit includes an oscillator tuned to the parameters at rest of the conductive winding associated with a capacitor, and a phase-locked loop, a voltage-controlled oscillator of which detects possible frequency variations in the rejector circuit formed of the conductive winding and of the capacitor.
According to an embodiment of the present invention, the excitation frequency of the rejector circuit ranges between 10 and 100 kHz.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.